Heretofore, there have been proposed various types of fingerprint sensors. A typical one of them is shown in FIG. 1. The fingerprint sensor is generally indicated with a reference 110. The fingerprint sensor 110 includes a sensor cell 100 formed from an array of sense electrodes 101 laid on the surface of a semiconductor, a matrix of row select lines . . . , 102n 1, 102n, 102n+1, . . . and column sense lines . . . , 103m 1, 103m, 103m+1, . . . , positioned correspondingly to the sense electrodes 101, and cell select switches Sr connected between each of the sense electrodes 101 and column sense lines . . . , 103m 1, 103m, 103m+1, . . . . The cell select switches Sr are selected row by row via the row select lines . . . , 102n 1, 102n, 102n+1, . . . .
As shown in FIG. 2, when a finger F is placed on an overcoat 104 covering the sense electrodes 101 of the fingerprint sensor 110, a capacitance Cs develops between the sense electrode 101 and the surface Fh of the finger F correspondingly to irregularities of the fingerprint. The capacitance Cs is sensed to recognize a fingerprint pattern. More specifically, when a portion of the finger corresponding to a ridge portion of a fingerprint is on the sense electrode 101, the capacitance Cs developed between the sense electrode 101 and the finger surface will be large since the distance between the finger portion and sense electrode 101 is short. On the other hand, when a portion of the finger corresponding to a valley portion of the fingerprint is on the sense electrode 101, the capacitance Cs will be small since the distance between the finger portion and sense electrode 101 is long. Thus, by sensing the capacitance Cs, it is possible to recognize a pattern of the fingerprint. It should be noted that a capacitance Cp in FIG. 2 is a parasitic one between the sense electrode 101 and a Si (silicon) substrate.
To sense a capacitance Cs, it has been proposed to charge a capacitance Cs at a constant voltage and sense a charge stored in the capacitance Cs. This method is known as a “voltage charging method”. A capacitance sensor adopting this voltage charging method is known from the Japanese Unexamined Application Publication No. 213908 of 2000, for example, which discloses a capacitance sensor in which a constant voltage is applied across a sense electrode and a charge stored in the sense electrode is converted into a voltage by a sensing circuit using an operational amplifier to sense a capacitance and to eliminate the influence of a parasitic capacitance between the sense electrode and a substrate, and there is provided a dummy electrode nearly identical in parasitic capacitance to the sense electrode to cancel the parasitic capacitance of the sense electrode.
In the above capacitance sensor, when there is not any object under detection on the sense electrode, that is, when no finger is placed on the sense electrode in case the capacitance sensor is used as a fingerprint sensor, or when a portion of the finger corresponding to a valley portion of a fingerprint is placed on the sense electrode, the sensing circuit senses a voltage Vsns as follows:Vsns=Vref{Cp×(Vc Vref)Cp′(Vref Vd)}/Cf  (1)where Cp is a parasitic capacitance of the sense electrode, Vc is a constant voltage applied to the sense electrode, Cp′ is a parasitic capacitance of the dummy electrode, Vd is a voltage applied to the dummy electrode, Vref is a reference voltage applied to the sensing circuit and Cf is a feedback capacitance of the sensing circuit (sense amplifier).
When the conditions Vc Vref=Vref Vd and Cp=Cp′ in the expression (1) are satisfied, the sensed voltage Vsns will coincide with the reference voltage Vref (i.e., Vsns =Vref). To meet these conditions, it is necessary to apply a highly accurate voltage across the sense electrode. If the sensing circuit has an offset, the expression (1) will not be met.
Note that since the parasitic capacitances Cp and Cp′ include a junction capacitance of the semiconductor as well and depend upon the bias, it is difficult to assure parasitic capacitances Cp and Cp′ accurately equal in value to each other. Therefore, when no finger is placed on the sense electrode or when the finger is placed at a valley portion of the fingerprint on the sense electrode, for example, it is extremely difficult for all chips (fingerprint sensors) to provide the same no-signal output level (will be referred to as “air level” hereinafter).
In the fingerprint sensor, when a capacitance exists between the surface of a finger and the sense electrode, the output signal level will vary depending upon the surface condition of the finger and thus the variation of the output signal level is allowable to some extent. Therefore, the fingerprint sensor has normally a function to adjust the output signal level to correct its variation. Since the air level itself is taken as a reference, however, the allowable range of the variation in output signal level from one chip to another is limited.